Most computer operations are executed in the arithmetic and logic unit (ALU) of the processor. Consider a typical example: Suppose two numbers located in the memory are to be added. They are brought into the processor, and the actual addition is carried out by the ALU. The sum may then be stored in the memory or retained in the processor for immediate use.
Any other arithmetic or logic operation, for example, multiplication, division, or comparison of numbers, is initiated by bringing the required operands into the processor, where the operation is performed by the ALU. When operands are brought into the processor, they are stored in high-speed storage elements called registers. Each register can store one word of data. Access times to registers are somewhat faster than access times to the fastest cache unit in the memory hierarchy.
The control and the arithmetic and logic units are many times faster than other devices connected to a computer system.